1. Field of the Invention
The present invention relates to a current balance technology, more particularly, to a passive current balance driving apparatus.
2. Description of the Related Art
Recently, with booming development of the semiconductor technology, portable electronic products and flat panel displays are widely used. Among various kinds of flat panel displays, the liquid crystal displays (LCDs) having the features of low operation voltage, no radiation, light-weight, small-size, etc. become a mainstream of display products. In general, since the LCD panel does not have a self-illuminating property. Therefore, the backlight module should be disposed under the LCD panel so as to provide a backlight source to the LCD panel.
A conventional backlight module can be roughly classified into two types, i.e. a cold cathode fluorescent lamp (CCFL) backlight module and a light emitting diode (LED) backlight module. Since the LED backlight module is capable of improving color gamut of the LCD, panel manufacturers prefer to employ the LED backlight module in replacement of the CCFL backlight module.
The LED backlight module has a plurality of LED strings connected in parallel, and each of the LED strings is composed by a plurality of LEDs connected in series. In practical, a DC-DC converter is mostly used to convert a DC power supply to a DC voltage so as to simultaneously drive each of the LED strings. However, since the characteristics of each of the LEDs are not exactly the same, for example, the internal resistance of the LED. Therefore, the total equivalent internal resistances of each of the LED strings are different. Accordingly, it can be inferred that the currents respectively following through each of the LED strings are not the same, namely, current unbalance. Thus, the brightness of the backlight source provided by the LED backlight module for LCD panel is non-uniform.
For solving such problem, a current balance control circuit can be added into the LED backlight module to regulate the currents respectively following through each of the LED strings, so as to make that the currents respectively following through each of the LED strings are the same, namely, current balance. A popular current balance control circuit may be composed by a current mirror circuit or a current feedback compensation circuit both constituted by a plurality of active elements such as MOSFETs. In addition, the current balance control circuit also can be composed by a ready-made current regulation chip such as a current sink IC.
However, the semiconductor devices such as the MOSFETs are always affected by process variation, and thus causing that the characteristic curves (Id/Vgs) for the drain current (Id) and the gate-source voltage (Vgs) of each of the MOSFETs are different. Therefore, the accuracy for using the current mirror circuit or the current feedback compensation circuit both constituted by MOSFETs to regulate the currents respectively following through each of the LED strings is limited.
In addition, the number of regulation driving channels of the ready-made current regulation chip is generally fixed, and the number of regulation driving channels is commonly 4 or 6. Since one regulation driving channel is only used for regulating the current following through one LED string, so that when the LED backlight module has 10 LED strings, three current regulation chips each having 4 regulation driving channels, or two current regulation chips each having 6 regulation driving channels should be elected and used, but whatever one is elected and used, two regulation driving channels are wasted. Therefore, there would be unnecessary costs.